Milling cutters are rotatable tools of cylindrical, conical, shaped or disk form, having a plurality of cutting edges. Such cutters are available in many forms, such as plain cylindrical, side milling cutters, face and end mills, formed cutters, and standard and special shaped profile cutters. High speed steel cutters are used for short production runs, carbide cutters are often used for long runs. One form of a cutting insert is described by Satran in U.S. Pat. No. 5,685,670. Similarly for lathe tools, each cutting edge has a clearance angle which is always positive, and a rake angle which is often positive, but may be zero or negative, for example when the cutter tooth is made of a hard grade of tungsten carbide and machining is carried out at high speeds yet without a coolant. Also similar to lathe tools, the recommended relief angles and rake angles depend both on the material to be machined and the material of which the cutter is made.
Much experimentation in the course of about a century has been carried out in an effort to find the best tool tip angle for milling metals. A tool tip that has too small an included nose angle will fail to dissipate heat and quickly reach temperatures causing softening and a sharp reduction of operating life, and/or tool failure. Also, such a tool is liable to vibrate, generate noise and may even break. Conversely a tool tip having too large a cutting angle may fail to cut without the application of high cutting forces. Both too large and too small a tooth tip angle can cause the production of a poor surface finish. Standard textbooks, such as “TOOL ENGINEERS HANDBOOK” and “MACHINERY'S HANDBOOK” provide tables of recommendations for these angles, these being based on much experience and practical tests.
Much research has been carried out to determine the largest possible volume of metal removed before tool failure, in relation to a chosen cutting speed. There are however so many additional factors involved, such as workpiece machinability rating, which itself is a function of both material type and heat treatment, tooth form, cutter size, number of teeth and cutter material, machine tool power available at the cutter and machine tool rigidity, cutter rigidity, coolant type and flow rate, surface finish required, feed rate chosen, and depth and width of cut that results published for one application are difficult to relate to other applications even where the basic type of work, e.g. milling, is the same. It is however clear that tool tip heating Is detrimental to long tool life, and anything that can be done to reduce the temperature of the tool tip will bring its reward in increased tool life.
A type of cutter used extensively is, for example, the cylindrical high speed steel and solid carbide end mill, which usually has helical teeth and a tooth face having a rake angle in the range of for example 150 degrees. The cutting face, as viewed in cross-section is usually a single concave curve extending without break from the tooth root to the cutting edge. A disadvantage of this tooth form is that there occurs extensive rubbing of the chip against the tooth rake face resulting in high power consumption, and the production of more heat than necessary which causes tool softening. It is of course the function of the liquid coolant to remove such heat, but studies have shown that the coolant never reaches the actual cutting edge which is most in need of cooling. In practice the coolant removes heat from the body of the tool and from the workplace, and heat is transferred by conduction from the hot cutting edge to the tool body. Tool steels are only moderately good heat conductors, so there is often a problem of too short a tool life due to a hot cutting edge.
A further problem is often encountered when using an end mill to machine a closed slot in ductile materials such as aluminum, copper, mild steel and brass in their annealed state. Chips do not clear easily out of the space between the milling cutter teeth, despite the fact that, as opposed to lathe tools, milling cutters always produce discrete chips. The conventional tooth shape previously mentioned is not helpful with regard to chip clearance.